Western Tibet – Massive collapse of 2 Glaciers in 2016 following
Surge-like Instability

Surges and glacier avalanches are expressions of glacier instability,
and are among the most dramatic phenomena that are known from the
mountain cryosphere. The catastrophic collapse of a glacier, that
combines the large volumes of surges and mobility of ice avalanches has,
until now, been reported only for the detachment in 2002 of the Kolka
Glacier in the Caucuses Mountains, totalling 130 x 106 x m3,
which has been considered to be a globally singular event. In this paper
Kääb et al. report on the
similar detachment of the entire parts of 2 glaciers adjacent to each
other in western Tibet in July and September 2016, which led to an
unprecedented pair of giant ice avalanches of low-angle with volumes of
68 ± 2 x 106 x m3 and 83 ± 2 x 106 m3.
Kääb et al. found, based on
satellite remote sensing, numerical modelling and field investigations,
that the cause of the 2 collapses was climate-driven and weather-driven
external forcing, which acted specifically on specific poly-thermal and
soft-bed glacier properties. Surge-like enhancement of driving stresses
and massively reduced basal friction which was connected to subglacial
water and fine-grained bed lithology, factors that converged to produce
the enhancement, to eventually exceed collapse thresholds in resisting
forces of the tongues frozen to their bed. It was found by Kääb et al.
that large catastrophic instabilities of low angle glaciers can happen
under rare circumstances without historical precedent.

Sudden mass failures of glaciers, that were gravity driven, have been
observed over a wide range of magnitudes, from ice falls at steep
glacier fronts, to large ice avalanches when hanging glaciers, typically
with angles steeper than 30o, detach (Faillettaz, Funk &
Vincent, 2015; Huggel, 2009). Impacts are mostly felt within a couple of
kilometres, with the exception of when the ice transforms to a mass
flow, which is highly mobile, by ingestion and production of meltwater
and the incorporation of sediments (Evens et al., 2009; Evans & Delaney,
2015). Topographic, atmospheric and ice-thermal conditions (Faillettaz,
Funk & Vincent, 2015; Huggel, 2009),and instabilities of underlying
bedrock or seismic events (Evens et al., 2009;van der Woerd et al., 2004) are typically included among the
factors which lead to pre-failure conditions and triggering.

According to Kääb et al.
glacier surges are a 2nd process of instability of a glacier,
which refers to events that last for weeks to a few years that have flow
speeds that are abnormally high, that can reach up to 10s of 100s of
metres per day over large parts of glaciers (Harrison & Post, 2003;
Yasuda & Furuya, 2015; Harrison et al., 2015; Sevestre & Benn, 2015;
Murray et al., 2003). Such surge-type glaciers are known of in clusters
from mountainous regions around the world, including the Tibetan Plateau
(Yasuda & Furuya, 2015; Sevestre & Benn, 2015; Jiskoot, 2011).

A 3rd type of glacier instability involves larger sections
that detach from low-angle valley glaciers. The Kolka Glacier event, the
Kazbek massif, Caucuses is the only such process that had previously
been documented in 2002, when 130 x 106 m3 formed
an ice/rock avalanche that travelled 18 km down the valley claiming 120
lives (Evans et al., 2009; Huggel et
al., 2005; Haeberli et
al., 2004). This picture was
changed by the massive glacier collapses in 2016 in Tibet (Tian et al.,
2017) and opened up critical questions about the causes of detachments
and the potential for similar events to occur elsewhere. In this paper
Kääb et al. describe the twin
events in Tibet and reconstruct the evolution of the collapsed glaciers
since the 1960s, based on remote sensing and mass-balance modelling.
Also, for more recent years they analysed glacier dynamics and modelled
the thermal conditions to infer details of the stress on the glacier and
frictional regime. Here they discuss the influences of melt and
precipitation, lithology of the bed and geometry of the glaciers on the
collapses.

From glacier collapses to high-speed avalanches

A massive volume of glacier ice detached from the lower part
(5,800-5,100 m above sea level) of a glacier that had not been named in
the Aru Range (Rutok County, China) in the western Tibetan Plateau, on
17 July 2016, 11:15 Beijing time, which has been termed Aru-1 for the
glacier and the collapse. The fragmented ice mass ran out 6 km beyond
the terminus of the glacier, and killed 9 herders and hundreds of their
animals, reaching the Aru Co lake (Tian et al., 2017) (⁓4,970 m above
sea level). The avalanche ran for 8.2 km and its vertical path of 800 m
yield an angle of reach that is surprisingly low of only 5-6o
(mobility index fahrböschung (Heim, Bergsturz & Menscheneben, 1932)),
which indicates very low basal friction in the movement. An area of 8-9
km2 was covered by the debris and Kääb et
al. used pre/post collapse
differencing of digital elevation models (DEMs) to calculate a volume of
the detached part of the glacier of 68 ± 2 x 106 m3.
An impact wave was generated by the avalanche which inundated the
opposite shore of Aru Co over a stretch of shoreline measuring 10 km,
and extended up to 240 m inland and 9 m above the level of the lake.

A second glacier (Aru-2) detached 2.6 km south of the July event on 21
September 2016. This second detachment occurred at 5,800-5,240 m above
sea level in 2 main flows at about 5:00 and 11:20 Beijing time. The
glacier mass fragmented and transformed into a mass flow that ran out 5
km beyond the terminus of the glacier, reaching 4,965 m above sea level
in a similar manner to the July event. The vertical height of the path
was 830 m over a horizontal distance of 7.2 km, which gave a similarly
low fahrböschung of 6-7o. In the 2nd collapse the
glacier debris covered 6-7 km2 with a volume of the detached
glacier of 83 ± 2 x 106 m3.

The geomorphology of the avalanche paths and deposits, based on
eyewitness reports, and observations of seismic waveforms that were
generated by the mass movements all suggest that the events occurred
suddenly and rapidly, with a duration of 2-3 minutes at a mean speed of
30-40 m/sec). The analysis of the 3-D forces inverted from the glacier
avalanche seismic signals reveals basal friction coefficients that were
exceptionally low, of 0.11 ± 0.07 for Aru-1 and 0.14 ± 0.05 for Aru-2
(ratio between frictional and normal force). According to Kääb et
al. these values are
exceptionally smaller than values on the order of 0.2-0.6 that had
previously been found for landslides. Kääb et
al. found average flow speeds
of ⁓20 m/sec, using different avalanche flow models (Christen, Kowalski
& Bartelt, 2010; Hungr & McDougall, 2009), which peaked at 70-90 m/sec
in the gorge sections of the paths. The presence of a grassy understorey
vegetation on the lee side of hills in the avalanche path of Aru-1 that
had not been destroyed also indicated that the avalanche speeds were
high, which suggests that the fast-moving mass had partially jumped over
it (Huggel et al.,2005). Processes of ice liquefaction from frictional
heating during the collapse and flow are suggested by the high speeds of
Aru-1 and Aru-2 avalanches (Schneider et al., 2011). Kääb et
al. suggest that up to 2-3%
of the ice could have liquefied if all potential energy was consumed,
based on simple energy conservation, with the water that was generated
being concentrated at the bottom of the flow.

Crevasses that were very similar to the ones preceding the Aru-1 event
were detected on satellite imagery prior to the collapse of Aru-2, the
potential avalanche run-out geometry and general pattern of deposition
were modelled from the Aru-1 simulation, and an alert was issued to
Chinese authorities.

Climate and glacier changes 1961 to 2016

Kääb et al. investigated
recent climate and meteorological records in the Aru region in order to
identify what possible pre-conditions and triggers could have caused the
2 glacier collapses. The closest long-term record was provided by the
Shiquanhe meteorological station, and these records revealed strong
warming of 1.7 ± 0.5oC since 1965, which is in line with
length of the glaciers in the western Tibet Plateau (Yao et al., 2012;
Ye et al., 2017). A long term trend in precipitation at Shiquanhe has
not been found, though in 2013 and 2015 there were exceptionally large
precipitation sums, and in 2016 at the Nagri station, which was
established in 2010.

Kääb et al. found by using
Corona satellite imagery for the period 1961-1980, Landsat from 1982
onwards, and high resolution optical and radar satellites from 2011 to
date, that the glaciers in the Aru Range retreated from the 1970s
onwards; Aru-1 glacier retreated by 520 m and the Aru-2 glacier by 460 ±
15 m between 1970 and 2015 or 2016. Contrasting with this frontal
retreat, it has been found by analysis of DEMs from STRM-X (2000),
TanDEM-X (2011-2016) and Advanced Spaceborn Thermal Emission and
Reflection Radiometer (ASTER)
(Brun et al., 2017), as well as ICESat laser altimetry (Kääb et
al., 2015) (2003-2008), that
glaciers in the wider Aru region are part of the Karakorum, Kulun Shan
and Eastern Pamir anomaly (Brun et al., 2017; Brun et al., 2017) and
experienced a slight increase in thickness of between 0.20 ± 0.16 m/yr
in the ASTER and 0.28 ± 0.15 m/yr in the ICESat, water equivalent (w.e.)
since the early 2000s. Steepening of the surface of the glacier is
caused by simultaneous thickening at high elevation and the thinning at
low elevation, which in generally not typical. For glaciers of the
non-surge type, however, it has been observed when there is an increased
temperature and snowfall simultaneously (Berthier et al., 2010) that
causes increased ice and melting and accumulation. Specifically,
non-surging glaciers in the Aru region thickened by up to 0.40 ± 0.15
m/yr above an elevation of about 5,650 m above sea level and thinned up
to 1.20 ± 0.15, m/yr below that elevation. In the range north of the Aru
Range for at least 5 of 16 glaciers this overall climate-driven pattern
of surface elevation changes was interrupted by mass-type
redistributions from 2000 to 2016.

Mass-balance modelling of the Aru glaciers confirms positive mass
balance between 1995 and 2008, and steepening of the mass-balance
gradient, despite regional warming. Kääb et
al. attributed this to an
increase in precipitation since the mid-1990s (Tao et al., 2014; Kapnick
et al., 2014) captured by the ERA-interim reanalysis that drives their
model. Widespread growth of the endorheic lakes of the region during the
same period confirms this increase in precipitation (Zhang et al.,
2017).

Searching the archives from Corona (from 1961) and Landsat (from 1972)
Kääb et al. found no
indication of glacier collapses having occurred earlier in the Aru Range
as well as its wider region (radius of about 300 km). Kääb et
al. noted however, that early
images from Corona and 2015/2016 satellite data for the 2 Aru glaciers
and some of their close neighbours show similar crevassing that is
strong and bulged tongues; i.e., features that commonly presage
surge-like instabilities or rapid advance.

Pre-collapse mass distribution within polythermal glaciers

In the period immediately before the collapse, 2011-2015, patterns of
elevation changes in the glacier surface are measured from repeat
TanDEM-X DEMs and optical satellite stereo DEMs.

The sections immediately above the eventual detachment zones of both Aru
glaciers at ⁓5,800-5,400 m above sea level bulged upwards. As early as
2011, at least, and until 2014, the sections immediately above the
eventual detachment zones at ⁓5,800 m of both Aru glaciers subsided.
Simultaneously glacier sections ⁓5,800-5,400 m above sea level bulged
upwards. Combined with a loss of thickness at the termini ⁓5,400-5,200 m
above seas level, the bulge resulted in steepening by 5-6o at
its front, and therefore to locally increasing driving stresses. It is
indicated by the changes in elevation rates derived for 2011-2014 that a
down-glacier mass transfer had begun already 2003 ± 3 yrs. However, this
internal mass movement did not result in surging of Aru-1 before
mid-2015 and an advance of only 200 ± 15 m up to collapse, whilst no
advance had been detected in Aru-2 at all. It was indicated by these
observations that strong resistive forces to sliding at the termini of
the glaciers, which suggests that they were frozen to their beds (Frappe
& Clarke, 2007).

It was revealed by offset-tracking that the medial bulge of Aru-1 was
associated with increased velocities of ice, from 0.18 ± 0.03 m/day in
late 2013, to 0.50 ± 0.04 m/day in the spring of 2016. According to Kääb
et al. these are 3-10 times
higher flow speeds compared to the flow of 0.05 m/day that had been
modelled. They found that average central speeds were unchanged in the
medial bulge of 0.12 ± 0.03 m/day between July 2013 and April 2016.

Crevassing that was strongly enhanced developed on both glaciers in the
weeks to months prior to the detachments. Along the lateral margins and
across the glaciers crevassing was particularly evident at an elevation
of about 5,800 m above sea level, which increased displacement where the
glaciers later detached. Cracks across the entire Aru-1 glacier had
already evolved in September 2015. On Aru-2 crevasses that were
developing rapidly were discovered on satellite images prior to its
collapse.

It is indicated that enhanced basal sliding occurred in the central
parts of the glaciers, based on the high initial acceleration of the
avalanches, the pattern of changes on the surface of the glaciers,
thermo-mechanical modelling of the glaciers, and the formation of
crevasses. It is also suggested that there were large amounts of water
by a fan that was mud-flow like of basal fines that originated at about
5,500 m above sea level from crevasses, which is observed on satellite
images of Aru-1 obtained 2 days prior to its collapse. No such
observation was made for Aru-2.

It is indicated by thermo-mechanical modelling that the interface
between the ice and the bed was probably temperate (thawed) in the
central part of the glaciers though cold (frozen) in the remaining
zones, which suggests a structure for the Aru glaciers that is
polythermal. The late (Aru-1) or missing (Aru-2) advances of the fronts,
significant sticking of the ice masses at the margins of the glaciers
following detachment, and the location of the Aru Range within a
semiarid region of permafrost support the thermal patterns that are
model-indicated (Gruber, 2012). The polythermal structure of the
glaciers led to conditions for:

·
Margins and fronts of the glacier tongues to be frozen to the underlying
bed;

·
Infiltrating melt water to reach the bed of the glacier at the top of
the detachment zones;

·
The retention of this water upslope of the cold-based plugs leading to
the precursory decrease in friction within the medial bulges (Clarke et
al., 1984);

·
And the development upstream of the cold-based fronts of a progressively
steepening geometry.

Critically, the geometry of the glacier was prevented by the cold-based
front from adapting quickly enough to the reduced friction, as would be
the case in typical surges. The accumulation of water at the bed of the
glacier through ingress of rainfall and summer melt in particular was
probably accelerated after 2010 when there was an increase in the sum of
both contributions by about 50%.

Causes and implications

Kääb et al. found that there
were no earthquake events that were associated with either of the Aru
events. It was indicated by Global Precipitation Measurement Integrated
Multi-Satellite Retrievals (GMP IMERG) and the meteorological stations
in the region that there was significant amounts of precipitation
through the summer of 2016 of up to a total of 200 mm or more. From 10
to 25 mm of rainfall was observed for 2 days immediately prior to the
Aru-1 event. Significant reduction of snow cover was revealed by optical
satellite images in the weeks leading up to the collapse. It is
indicated by very low back-scatter in the Sentinel-1 radar data from 1
July through to 31 August 2016 that continued melting conditions up to
the tops of the mountains of the Aru Range (about 6,100 m above sea
level). Several rainfall events occurred during the weeks leading up to
the collapse, though there was no similar strong precipitation observed
for the days immediately preceding the Aru-2 event. Therefore, at least
for Aru-1, unusually high liquid water from melting snow and
precipitation (highest positive degree-day summer since 1979 found for
2016) increased the content of water in the glacier system, which
therefore puts it among the possible triggering factors for the
collapse.

The deformed bed (Harrison & Post, 2003; Harrison et al., 2015; Truffer
et al., 2000; Boulton & Jones, 1979; Cuffey & Paterson, 2010) of the Aru
glaciers is an important factor linked to the fine-grained sandstone and
siltstone, that is possibly lightly metamorphosed, that was mapped
beneath the collapsed Aru glaciers, and till rich in clay, which was
sampled. Kääb et al. suggest
that on commencement of basal thaw and sliding, the till that was formed
from these lithologies,
combined with the high input of water, may have formed basal slurries
locally that had low shear strength under high pore-fluid pressure
(Kamb, 1991), and may possibly have destroyed any subglacial drainage
system that was present (Clarke et al., 1984). In those areas the
limited strength of the till would have induce d a rheology prone to
trigger instabilities (Tulaczyk, Kamb & Engelhardt, 2000; Iverson,
Hooyer & Baker, 1998).

The 2 tongues of the glaciers might have also influenced their stability
by causing lateral resistance because of their curved shape in plan
geometry. This is indicated by strong crevassing and maximum flow speeds
at the northern, left-lateral glacier margins. Kääb et
al. also noted that the
surface slopes of 12-13o (bed slopes 9-10o) of the
detached avalanche sources rank rather high for glacier tongues of surge
type, which would therefore exert high driving stresses (Harrison et
al., 2015).

The foregoing shows, in sum, that three is no evidence for a single
trigger for the twin collapses. The factors that are present in a number
of other glaciers on the Tibetan plateau, as well as elsewhere, where:

·
Surge-like behaviour;

·
Glacier steepening that is driven by climate;

·
Polythermal glacier structure’

·
Their geometry and slope;

·
Liquid water from melting snow and rain during the summer of 2016.

Kääb et al. proposed
therefore that the collapse of the Aru glaciers was caused by a
transient convergence of factors that acted at different spatial and
temporal scales. Important basic conditions are represented by a
polythermal glacier regime, glacier morphology and lithology of bedrock.
The simultaneous increase in temperature and precipitation over
timescales of 15-20 years has acted on the geometry of the glacier and
basal friction respectively increasing the slope and enhancing the input
of liquid water to the bed of the glacier, and possibly expanding the
area over which thawed conditions exist. The glaciers are prevented from
adjusting their geometries to changed driving stresses and frictional
conditions by the presence of cold-based fronts and margins. This
increased continuously the stress on the frozen terminus and margins,
until reaching a critical point at which the resisting forces were
eventually overcome and the collapse occurred. These events are made
unique by the sustained exceptional low frictions over large parts of
the glacier, and are probably associated with a particular response of
the fine-grained basal till to large amounts of liquid water (Harrison &
Post, 2003; Harrison et al., 2015; Clarke et al., 1984; Roering et al.,
2015). It is believed that the final trigger is linked to short-term
variation in subglacial hydrological system that was induced by
unusually high water input from melting snow and rain in the summer of
2016. Potentially, hydro-thermodynamic feedback also played a role – a
mechanism by which basal sliding is increased by water reaching the bed
of the glacier through crevasses, strain and fracturing that is
associated with it by a combination of thermal and dynamic changes,
therefore facilitating the input of additional water and modifying
subglacial drainage (Fountain et al., 2005).

There are some similarities between the Aru events in 2016 and the Kolka
Glacier collapse of 2002 including volume, glacier and avalanche slopes,
and speed of the avalanche. In the days leading up to the detachment
(Evans et al., 2009; Huggel et al., 2005; Haeberli, 2004), signs of
destabilisation were shown by the Kolka Glacier which included heavy
crevassing and bulging, as well as unusual hydrological conditions
(Fountain et al., 2005) such
as supraglacial ponds. The Kolka Glacier surged in 1969/1970 (Huggel,
2005) and had a slope that was similar to that of the Aru glaciers
(Evans et al., 2009). The Kolka/Kazbek area is known for its
fine-grained rocks and volcanic sediments (Drobyshev, 2006) which cause
a variety of mass movements (Chernomorets et al., 2007), and putting
forward the possible role of lithology in the rapid and sustained
reduction of basal friction in the 3 events. Beneath the Kolka
(Drobyshev, 2006) and Aru glaciers unusual geothermal heat fluxes are
not certain but they cannot be excluded. The Kolka Glacier, however,
contrasts with the Aru glaciers in having been temperate throughout.
Also, heavy rock and snow fall activity deposited several million m3
of material on the glacier over a number of weeks before the surge event
(Evans et al., 2009; Huggel et al., 2005; Haeberli et al., 2004). When
the Kolka andAru cases are considered
together 2 possibilities are suggested to reach critical glacier
geometry and an associated increase in driving stresses; i.e.,
additional loading from surrounding mass movements (Kolka) or steepening
of the glacier (Aru).

A new form of glacier instability, the catastrophic collapse of large
parts of an entire valley glacier, in this case about 25-30% by area,
and up to 40% by volume, has been recognised as a result of the twin
glacier collapses of the Aru glaciers. It seems that these collapses are
made possible by a rare, though not unique array of factors that
coalesce into an anomalous increase in driving stresses, as well as
rapid and sustained reduction of basal friction. Kääb et
al. say it is spectacular and
completely unprecedented that such combined exceedances of instability
thresholds, that are highly unlikely for a single glacier, occurred on 2
neighbouring glaciers within 2 months. It doesn’t seem likely that
rock-mechanical or hydrologic/hydraulic collateral effects of Aru-1
could have triggered, or preconditioned the collapse of Aru-2, which
highlights the role of common external forcing by climate and weather in
synchronising the 2 collapses. New light is shed on the occurrence of
glacier instabilities that are surge-related; several documented glacier
instabilities (Zhang, 1992; Ugalde et al., 2015; Heybrock, 1935;
Espizua, 1986; Milana, 2007) should be revisited to clarify their
relation to the processes of glacier collapse suggested here. It is
implied by the transient nature of some of the factors involved in the
collapses of the Aru and Kolka glaciers, and the range of conditions and
thresholds that are suggested by the differences between them, that such
events can happen without historical precedent. It has been suggested
that when the deposits from the Aru avalanche have melted there should
be an investigation on their geomorphic and lithologic imprint and a
search should be carried out in the wider region for signs of potential
previous glacier collapses. The analysis of Kääb et
al. has shown that under
specific circumstances climate variability and change have a critical
potential to contribute to glacier instability on a large scale by
changing glacier geometry, thermal conditions and the content of liquid
water. Though it seems beyond reach at the present that long-term
prediction of similar events could be achieved the pre-event
observations and simulations of the Aru-2 collapse demonstrate that
available, state-of-the-art ground-based and satellite monitoring, as
well as modelling capabilities allow early warnings to be issued even in
regions that are very remote. Scientific advances to benefit also the
most remote communities would be possible by harnessing these
capabilities.